H.E.S.S.

High Energy Stereoscopic System

The Supernova Remnant G0.9+0.1

VHE gamma rays from the pulsar wind nebula

February 2005

Radio map of the
supernova remnant G0.9+0.1, showing a bright core region and a partial shell
(Helfand
and Becker).

Radio images of the Galactic Center region (see
Dec. 04) include a number of shell-type supernova remnants.
G0.9+0.1
is a composite remnant, showing a partial shell - about 8' in diameter -
surrounding a bright core (see image above). Assuming that the object is located
at a similar distance as the Galactic Center, one can deduce an age of several
1000 years. The radio emission from the shell is presumably caused by
electrons accelerated in the supernova shock wave. The bright core - which is
also resolved as an extended structure in X-ray observations with
Chandra and
XMM
- is identified as a pulsar wind nebula, akin to the Crab Nebula (Oct.
2004). A highly relativistic flow of electrons and positrons accelerated by
the fields of the pulsar provides the energy source feeding a termination shock,
where particles are accelerated to even higher energies in turbulent magnetic
fields. A pulsar candidate (CXOU J174722.8-280915) is seen in Chandra images as
a point source near the center of the nebula, however no pulsed emission has
been detected.

Already the first observations
of the Galactic Center region with two H.E.S.S. telescopes in 2003 showed a
faint signal at the location of G0.9+0.1. In 60 h of follow-up observations
between March and September 2004 - now with all four H.E.S.S. telescopes - a
highly significant signal is detected (13 sigma), consistent with the position
of the core of the supernova remnant (Fig. 1). Within the
angular resolution of H.E.S.S., the source is point-like, with an emission
region smaller than 1.3' (rms). The location of the source and the size limit
strongly suggest the pulsar wind nebula as the source of the VHE gamma-ray
signal, rather than the supernova shock wave. Over more than a decade in energy
- between 200 GeV and 6 TeV - the energy spectrum of gamma rays follows a power
law with a spectral index of 2.4 (Fig. 2). At a flux
corresponding to 2% of the flux from the Crab Nebula and assuming a distance of
8.5 kpc, the total power radiated in VHE gamma rays is 2x1034 ergs/s,
roughly half of the power output of the Crab Nebula, the only other established
VHE source of this type.

Indeed, the wide-band energy spectra (Fig.
3) are well modeled assuming a primary population of electrons, which emits
synchrotron radiation in the radio- and X-ray bands, and which creates the VHE
gamma-rays by scattering ambient low-energy photons to high energies. Close to
the Galactic Center, starlight photons represent to dominant source of target
photons. At higher gamma-ray energies, the Klein-Nishina effect starts to
suppress interactions with the (eV) starlight, and the photons of the cosmic
microwave background take over, effectively adding up to a smooth power low
spectrum in the H.E.S.S. energy range.

References:

Fig. 1:
Smoothed map of the VHE gamma-ray counts detected by H.E.S.S. (Color scale),
with radio contours superimposed. The cross indicates the best estimate for
the location of the TeV source, the dashed circle the limit on the (rms)
source size (assuming a Gaussian source distribution).

Fig. 2:
Energy spectra of VHE gamma rays detected with H.E.S.S., using the regular
cuts to select gamma rays (closed symbols) and special cuts to improve the
signal-to-noise at high energies (open symbols).